1. Field of the Invention
The invention relates to a system for transmitting video pictures by means of a hybrid encoder for encoding the data of an incoming video picture in blocks, the data of the previous video picture being stored in a picture memory of the hybrid encoder and being compared blockwise with the blocks of the incoming video picture, the result of the comparison for each block to be encoded leading to main and side information, the main information comprising data about the elements of a transformed and quantized block and the side information comprising data about the following block attributes:
a) the transformed and quantized block is the difference block or the original block,
b) the motion vector is zero or not zero,
c) the main information consists of zeros only or does not consist of zeros only.
If desired, said system may further comprise a hybrid decoder for performing operations reverse to the encoding steps of the hybrid encoder.
2. Description of the Related Art
A system having the above-mentioned functional features is described in the document #141 of CCITT of September 1986 (compare CCITT SGXV, Working Party XV/1, Specialist Group on Coding for Visual Telephony, document #141, Sept. 12, 1986, p. 5).
The main purpose of a hybrid encoder for such a system is to encode the video data coming from a video data source with a minimum possible loss of information into a signal with a minimum possible bit rate.
In this process two encoding principles - hence the name hybrid encoder - are used:
the interframe principle
in which the correlation between temporally sequential video pictures is utilized (this designation is used for full pictures and sub-pictures), and
the intraframe principle
in which the correlation of the video data within a video picture is utilized.
Before the actual encoding process, it is necessary to prepare the video data:
The video data are applied in so-called blocks to the encoder. Such a video data block (hereinafter also referred to as block or data block) comprises the data of given picture elements of a video picture which are considered to be elements of a quadratic numeral matrix. For example, a video data block may consist of the chrominance values of the first eight picture elements of the first eight lines of a video picture. Each video picture is split up into equally large data blocks.
The 8.times.8 video blocks of the incoming video picture are compared by means of the known hybrid encoder with corresponding blocks of the previous picture which are stored in a picture memory. The result of the comparison - the result block - is either the difference between the two blocks or the block of the incoming video picture. Details will be given hereinafter.
The block of the previous video picture corresponding to a block of the incoming video picture is herein understood to mean the block having the greatest conformity with the block of the incoming video picture.
This may be that block of the previous video picture which has the same location as the incoming block in the video picture, or a block which is displaced with respect to the location of the incoming block. The latter case often occurs in video pictures in which persons or objects move in front of a steady background. The displacement is indicated by a motion vector. This vector may also be the zero vector; in that case corresponding blocks have the same location in successive video pictures. The result block (the difference block or the original block) is then subjected to a Fourier transform (compare, for example, German Patent Application P36 13 393.4) and is subsequently quantized. If no element of the block is significantly different from zero after the quantization, the block is not transmitted. If the value of an element of the block is at least higher than a threshold, a Huffman encoding is performed and the values of the block are written as main information in a buffer memory. The data of successive blocks are separated by an END-OF-BLOCK symbol.
The following so-called side information is written in an encoded form in the buffer memory:
1. The motion vector is zero or not zero PA0 2. The transformed and quantized block is the difference block or the original block; PA0 3. The main information consists of zeros only or does not consist of zeros only; PA0 4. The components of the motion vector; and PA0 5. The magnitude of the quantization intervals of the adaptive quantizer.
Main and side information of the blocks are then read from the buffer memory with a temporally constant bit rate and transmitted to a receiver, and this in such a way that the receiver with its hybrid decoder is capable of splitting up the information relating to a block into main and side information and of reversing the individual encoding steps.
The block size, i.e. the number of picture elements, combined in a quadratic block is conventionally a power of two in video picture encoding. Blocks having a size of 8.times.8 or 16.times.16, i.e. quadratic blocks with data of 64 or 256 picture elements are significant. It will be readily evident that the processing time of blocks increases as their number of elements increases. In the case of fast encoders, i.e. those envisaged for high transmission bit rates, large blocks will therefore be more unfavorable than smaller blocks due to their considerable processing time. On the other hand, the ratio between main and side information will be in favor of side information in the case of small blocks, which reduces the encoding efficiency. The block size is thus the result of a compromise.
If a system of the type described in the opening paragraph were used for 8.times.8 blocks, for example, for a transmission bit rate of 64 kbit/s and a conventional picture frequency of 10 Hz, the transmission bit rate would not even be sufficient for transmitting the occurring side information, as will be elucidated hereinafter.